Normally tacky and pressure-sensitive adhesive (PSA) materials have been used for well over half a century. Products of this type, which take the form of tapes, labels, and other types of adhesive coated sheets, must be protected from unintended adhesion to other surfaces. Hence, tapes are typically wound into a roll on their own backing and labels are typically laminated to a release sheet to prevent their accidental adhesion to other surfaces and also to prevent their contamination with air-borne dust and other contaminants. In order to allow the roll to be unwound without the undesirable transfer of adhesive to the tape backing, it is customary to provide the tape backing with a low adhesion backsize (LAB). Similarly, the release sheet or liner to which the adhesive coated label is typically laminated is supplied with a release coating to permit the easy removal of the liner from the label.
This LAB or release coating is expected to reproducibly provide an appropriate level of release from the adhesive of interest, to not deleteriously affect the adhesive, and to be resistant to aging so that the release level remains relatively stable with time. In recent years, as competition has expanded in the PSA industry, a need to differentiate product performance as well as more demanding product requirements has lead to recognition of the importance of release level. D. Satas, Chapt. 23 "Release Coatings", Handbook of Pressure Sensitive Adhesive Technology, Second Edition, D. Satas, ed., Van Nostrand Reinhold, 1989, defines seven distinct levels of release, ranging from "super low release" (0.15-0.30 N/dm) to "very tight release" (20-80 N/dm). Even within a given category, such as "moderately tight release" (6-10 N/dm), consumer preference demands a tighter unwind for a roll of office tape in Europe and Japan than in North America, allowing the manufacturer who has a means of easily adjusting the level of release the opportunity to compete on a global basis.
In many circumstances it is important for the LAB to possess other properties besides functioning as a release agent. For example, the release coating on masking tape must possess good solvent resistance in addition to providing a surface to which paint can adhere.
Long chain linear and branched hydrocarbon polymers find widespread use as low adhesion backsizes for pressure sensitive tapes. The alkyl side chain of these acrylate (U.S. Pat. No. 2,607,711), methacrylate (U.S. Pat. No. 3,502,497 and U.S. Pat. No. 4,241,198) vinyl ester (U.S. Pat. No. 2,829,073), vinyl carbamate (U.S. Pat. No. 2,532,011), etc., copolymers apparently crystallizes to form a waxy low energy surface to which the adhesive adheres poorly. These various polymeric release coatings are not universal, in that none of them show desirable release performance for every type of P$A. In addition, the range of release level possible with these polymers is fairly limited.
Fluorocarbon copolymers also provide low surface energy coatings which find utility in certain specialty applications (U.S. Pat. No. 3,318,852), but also lack universality and release level tailorability.
Silicones are widely used for release liner applications due to the fact that they provide easy release for a wide variety of PSA types. Silicones are generally less useful as LABs in tape constructions, however, where tighter release levels are desirable. To increase their usefulness in LAB tape constructions, organosiloxane has been blended with a variety of film forming coating materials such as nitrocellulose (U.S. Pat. No. 2,985,554), alkyl ether-maleic anhydride copolymers (U.S. Pat. No. 3,770,687 and U.S. Pat. No. 3,823,025), vinyl alkyl carbamate copolymers (U.S. Pat. No. 3,679,458), etc., to tighten release. In addition, silicones can be modified with epoxy groups to make them more polar (U.S. Pat. No. 4,313,988) hence providing tighter release. Silicones can also be cocured with isocyanates (U.S. Pat. No. 3,957,724), polybutadiene (U.S. Pat. No. 4,261,876), acrylic emulsions (U.S. Pat. No. 3,933,702), etc., to tighten release.
Release coating compositions based on polysiloxane grafted copolymers that do not require a curing step have been described (U.S. Pat. No. 4,728,571) wherein controlled and predictable release is achieved through variation in the number and the length of the polysiloxane grafts.
Polymers having at least one polymeric siloxane segment and at least one hydrophilic vinyl polymeric segment prepared by other means than by use of the so-called "iniferter" technique have been shown to demonstrate utility as release coatings that are capable of being written on effectively with water-based and oil-based pen inks as described in copending U.S. patent application, Mertens, Ser. No. 07/278,283, filed Nov. 30, 1988 (assigned to the assignee of the present case), and now abandoned. Mertens, which is based upon a thermal method primarily relates to ink receptive copolymers. Mertens does not teach the preparation of ink receptive and non-ink receptive copolymers by use of iniferter technolgy and does not describe the broad utility of vinyl siloxane block copolymers for release coatings which are not ink receptive.
Various methods of preparing block copolymers of silicone and vinyl monomers have been described in the art. Crivello teaches a thermal method involving the use of a macromolecular siloxane initiator (U.S. Pat. No. 4,584,356) and describes utility as E-beam resists (U.S. Pat. No. 4,677,169) and positive or negative resists (U.S. Pat. No. 4,689,289). Eichenauer, et al describes condensation of either a semitelechelic hydroxy-terminal vinyl polymeric segment with an acetoxy-terminal siloxane (DE 3,606,984) (publication date Sep. 10, 1987) or a semitelechelic carboxy-terminal vinyl polymeric segment with an aminopropyl-terminal siloxane (DE 3,606,983) (publication date Sep. 10, 1987). Japanese laid-open applications 63-57642 and 63-57644 describe the use of peroxy ester terminated siloxanes as macroinitiators for thermal preparation of vinyl-siloxane block copolymers. H. Inoue, et al, J. Appl. Poly. Sci, 35, 2039 (1988) describes properties of poly(methyl methacrylate)/siloxane block copolymers obtained from thermal polymerization using an azo-containing siloxaneamide macroinitiator.
These methods, while useful, involve reactions that are not easily controlled. Since the reactions are not easily controlled, the copolymers formed thereby cannot be easily tailored. The block polymers prepared by these methods have not been shown to be useful for release coatings for pressure sensitive adhesives and in particular have not shown that a range of release performance can be achieved for a variety of PSA types.
An article by Noshay and McGrath, entitled "Block Copolymers", Academic Press, New York, 1977, pp 156 to 162, which is hereby incorporated by reference discusses phase separation as it applies to silicone-vinyl block copolymers. Noshay does not teach the preparation of vinyl-siloxane block copolymers by use of iniferter technology. Rather, Noshay teaches the preparation of vinyl-siloxane block copolymers by use of anionic polymerization and condensation polymerization methods.
The present invention provides sheet materials having release coatings of iniferter-prepared vinyl-siloxane block copolymers and tape constructions having low adhesion backsizes of the same copolymers. The iniferter-prepared block copolymers which can be reliably produced, exhibit specific release properties toward tacky and pressure-sensitive adhesives throughout a broad range.
The term "iniferter" refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent, and terminator, the term "iniferter" being a word formed by the underlined portions of the terms identifying these functions. This term and its use in the production of block copolymers is well known, particularly because of the work of Takayuki Otsu of the Department of Applied Chemistry, Osaka City University, Osaka, Japan. This work is discussed, for example, in an article by Otsu, et al entitled "Living Radical Polymerizations in Homogeneous Solution by Using Organic Sulfides as Photoiniferters", Polymer Bulletin, 7, 45-50 (1982), an article by Otsu, et al entitled "Living Mono- and Bi-radical Polymerizations in Homogeneous System, Synthesis of AB and ABA Type Block Copolymers", Polymer Bulletin, 11, 135-142 (1984), and in European Patent No. 0286376, published Oct. 12, 1988.
Copending U.S. application Ser. No. 07/212,594, Ali, et al., filed Jun. 28, 1988, (assigned to the assignee of the present case), and now abandoned, discloses the use of iniferter technology in the preparation of acrylic block copolymers having the requisite physical properties making them suitable for use in pressure-sensitive adhesive compositions. The control of the polymerization permits tailoring of the reinforced acrylic block copolymer to provide a balance of adhesion, cohesion, stretchiness and elasticity to make a successful pressure-sensitive adhesive. Copending U.S. application Ser. No. 07/212,594, filed Jun. 18, 1988, does not disclose siloxane iniferter compounds or the use of such iniferter compounds in the synthesis of vinyl-siloxane block copolymers.
Copending U.S. application Ser. No. 07/212,593, filed Jun. 26, 1988, Andrus Jr., et al., (also assigned to the assignee of the present case), now abandoned discloses the use of iniferter technology in the preparation of acrylic block copolymers which can be tailored to provide optical clarity and resistance to oxidative and photochemical degradation. The acrylic block copolymers disclosed in copending U.S. application Ser. No. 07/212,593 are employed to make shaped articles, sheet materials, and the like. Copending U.S. application Ser. No. 07/212,593 also does not disclose siloxane iniferter compounds or the use of such iniferter compounds in the synthesis of vinyl-siloxane block copolymers.
Despite the rather detailed description of making other block copolymers according to the above disclosures, there is no disclosure of the novel siloxane iniferter compounds or the vinyl-siloxane block copolymers made therewith claimed in copending U.S. application, Kumar, et al, Ser. No. 393,550, Siloxane Iniferter Compounds, Block Copolymers Made Therewith And A Method Of Making The Block Copolymers filed Aug. 14, 1989, now abandoned. Nor is there taught the use of such vinyl-siloxane block copolymers as release coatings for the coated sheet materials herein claimed.
Copending concurrently filed U.S. Application Kumar, et al, Siloxane Iniferter Compounds, Block Copolymers Made Therewith and a method of making the Block Copolymers, (also assigned to the assignee of the present case) incorporated by reference herein, teaches the utilization of a novel siloxane iniferter as a means of promoting, controlling and terminating polymerization of a vinyl-siloxane block copolymer.
The siloxane iniferters used in preparing vinyl-siloxane block copolymers useful as release coatings are macro "iniferters", as opposed to the iniferters of Otsu; Ali, et al, U.S. application Ser. No. 07/212,594; and Andrus Jr., et al., U.S. application Ser. No. 07/212,593, which are low molecular weight iniferters which do not contain a polymerized siloxane segment or any silicon for that matter. It is not apparent from the above references that a polymerized silicone or siloxane segment could be contained in an iniferter. Both the preparation and function of the "macro" siloxane iniferters useful in preparing vinyl-siloxane copolymers differ from the iniferters described in the references above.
The selection of the endblocker which can cooperate with the siloxane midblock segment to form the macro siloxane iniferter useful in preparing tailormade vinyl siloxane block copolymer is very critical and must be made in a way which will not diminish the effectiveness of the siloxane segment. The preparation of such a "macro" iniferter including the determination of useful endblocker in its preparation is not suggested by any of the above references.
The polymerized siloxane segment introduced into the "backbone" of the vinyl-siloxane copolymer by the photoiniferter polymerization technique is a midblock component. The endblocker, which caps the siloxane segment, is selected such that it reacts with the photoiniferter in order to form a "macro" siloxane iniferter compound. The bond between the photoiniferter and endblocker is broken upon exposure to ultraviolet radiation resulting in an initiator free radical and a terminator free radical. The initiator free radical is capable of polymerizing free radically polymerizable vinyl monomer in order to yield vinyl siloxane copolymer.
With respect to Ser. No. 07/212,594, Ali, et al., and 07/212,593, Andrus Jr., et al., both the midblocks and endblocks comprise polymerized acrylic monomers. It is not apparent that a polymerized siloxane segment could be included in an iniferter and polymer formed therewith. Otsu also does not teach "macro" siloxane iniferters or the preparation of vinyl-siloxane compolymers.
By using photoiniferter polymerization techniques, it is possible to obtain more complicated polymer architectures than are avaliable according to the method of Crivello. The thermally prepared block copolymers of Crivello cannot possess architecture beyond AB and ABA.
In addition, photoiniferter polymerization reactions are much more efficient than thermal polymerization methods in terms of providing cleaner block copolymers which are free from unwanted homopolymers. Moreover, photoiniferter polymerization methods can be used to form block copolymers ranging from very low to very high molecular weights. The photoiniferter method permits the tailoring of the siloxane midblock as well as the endblocks to satisfy customer needs which is not possible using Crivello's thermal method.
Release coatings and LABs are typically applied to their substrates at coating weights around 1 g/m.sup.2. In order to obtain these thin coatings, dilute solutions (2 to 5% solids) of the coating compositions in organic solvents have traditionally been used. Recent efforts have been directed to delivering such coatings at high or 100% solids or from aqueous media, thus reducing the environmentally damaging hydrocarbon emissions, conserving precious natural resources, and lowering economic cost.
U.S. Pat. No. 3,933,702 eliminates the organic solvent that is present in most LAB formulations by cocuring 100% silicones with acrylic emulsions.
A drawback however, is that release is not tailorable for these LAB formulations. In addition, most LAB formulations require curing and in addition are not ink receptive.
Moreover, such compositions have limited pot lives. In addition, the need to monitor the degree of cure is disadvantageous. The technology of converting solvent borne release formulations to water compatible systems for basic segmented silicone-urea block copolymers is discussed in copending U.S. application Leir et al Ser. No. 07/300,346, filed Jan. 23, 1989 (assigned to the assignee of the present case), now abandoned. Leir does not teach the conversion of solvent borne iniferter prepared vinyl-siloxane release formulations to water compatible systems.